A fuel cell capable of generating electric power with high efficiency even if the fuel cell is small has been developed as an electric power generator of a distributed energy supply source. However, means for supplying a hydrogen gas used as fuel during this electric power generation is not developed as a common infrastructure. Therefore, a hydrogen generator which carries out a steam-reforming reaction of a raw material obtained from an existing infrastructure, such as a city gas or LPG, to generate a hydrogen gas is typically disposed with the fuel cell.
A reformed gas containing the hydrogen gas obtained by the hydrogen generator through the steam-reforming reaction contains carbon dioxide and carbon monoxide derived from the raw material. Since the carbon monoxide deteriorates an electric power generation property of the fuel cell (especially, PAFC and PEFC) which utilizes the hydrogen gas, it is desirable that a concentration of the carbon monoxide in the reformed gas be decreased as low as possible. On this account, to decrease the concentration of the carbon monoxide, the hydrogen generator includes a shift converter section which carries out a shift reaction between the carbon monoxide and steam to generate the hydrogen gas and a purifier section which carries out a selective oxidation reaction between the carbon monoxide and an oxidizing agent, such as a very small amount of air, to oxidize the carbon monoxide. In respective reaction sections, a catalyst suitable for progressing the reaction is used. For example, a Ru catalyst and a Ni catalyst are used in a reformer section, a catalyst (hereinafter referred to as “copper-zinc catalyst”) containing copper and zinc as major components and a precious metal-based catalyst are used in the shift converter section, and the Ru catalyst and the like are used in the purifier section.
Since the shift reaction between the carbon monoxide and the steam in the shift converter section is an exothermic reaction, it is desirable to carry out the reaction at low temperature to effectively decrease the carbon monoxide. However, in the case of carrying out the reaction at low temperature, a reaction rate becomes low, so that a large amount of catalysts are necessary to progress the reaction. Therefore, in the case of using a precious metal-based shift catalyst, catalyst durability is easily secured, and a device operating condition is simple, however, since the use amount of the precious metal becomes large, the catalyst cost increases, which is a demerit.
In contrast, in the case of using the copper-zinc catalyst as the catalyst of the shift converter, since copper and zinc that are comparatively common metals are used as the raw materials, the catalyst cost becomes cheaper than the precious metal-based catalyst. However, the copper-zinc catalyst has a problem in the resistance to oxidation. For example, a catalytic activity of the copper-zinc catalyst tends to deteriorate due to the oxidation caused by air, steam or the like. Therefore, in the case of using a catalyst, such as the copper-zinc catalyst, which is easily oxidized by air or steam, a method for filling a catalyst existing space in the hydrogen generator with an inactive gas, such as nitrogen, or a material gas, such as a city gas or LPG, is adopted to maintain the catalyst existing space in a reducing state during stopping of the operation.
However, even in the case of adopting such method, it is difficult to secure complete sealing during stopping of the hydrogen generator for a long period of time. Since mixing of air into the hydrogen generator is unavoidable, the oxidation of the copper-zinc catalyst occurs, deteriorating a catalytic ability.
Known is a method for automatically reducing the shift catalyst oxidized in a shift reactor which has been left for a long period of time (see Patent Document 1 for example). Moreover, known is a reforming device which reduces the shift catalyst by a reducing gas generated by a combustor, and automatically determines the degradation of the shift catalyst and processes it (see Patent Document 2 for example).    Patent Document 1: Japanese Laid-Open Patent Application Publication Hei 10-64571    Patent Document 2: Japanese Laid-Open Patent Application Publication 2002-124286